Metal mold for hot pressing

ABSTRACT

A metal mold for hot pressing according to the present invention includes a fixed mold and a movable mold disposed such that molding surfaces of the fixed mold and the movable mold face each other. The metal mold sandwiches a heated workpiece between the fixed mold and the movable mold to press the workpiece while, at the same time, cooling the workpiece by keeping the molding surfaces of the fixed mold and the movable mold in contact with surfaces of the workpiece. A layer formed by a shape-memory alloy is formed at least at a portion, on which fitting during pressing is tight, of the molding surface of the fixed mold. According to the metal mold for hot pressing of the present invention, formation of a gap between the metal mold and the workpiece is suppressed and uniform contact between the metal mold and the workpiece can be achieved.

TECHNICAL FIELD

The present invention relates to a metal mold structure used for hot press molding for pressing a heated workpiece while, at the same time, cooling the workpiece.

BACKGROUND ART

Conventionally, there is widely known hot press molding (see PTL 1, for example). This hot press molding carries out pressing of a workpiece such as a steel plate heated to a temperature at which an austenite structure appears or a higher temperature by using a pressing machine mounted with a metal mold including an upper mold and a lower mold while, at the same time, carrying out quenching treatment utilizing cooling due to contact between the metal mold and the workpiece.

CITATION LIST Patent Literature

PTL 1: JP 2009-72801 A

SUMMARY OF INVENTION Technical Problem

In the conventional metal mold for hot pressing, wrinkles due to a shape of a molded product, manufacturing accuracy of the metal mold, a warp of the metal mold, a warp of the pressing machine, or the like causes a difference in fitting (tightness of fitting) of the clamped metal mold and the workpiece on each other. As a result, a gap is formed between the metal mold and the workpiece, which obstructs uniform cooling of the workpiece.

Therefore, an objective of the present invention is to provide a metal mold for hot pressing with which formation of a gap between the metal mold and the workpiece can be suppressed and uniform contact between the metal mold and the workpiece can be achieved.

Technical Solutions

A metal mold for hot pressing according to the present invention includes a fixed mold and a movable mold disposed such that molding surfaces of the fixed mold and the movable mold face each other. The metal mold sandwiches a heated workpiece between the fixed mold and the movable mold to press the workpiece while, at the same time, cooling the workpiece by keeping the molding surfaces of the fixed mold and the movable mold in contact with surfaces of the workpiece. A layer formed by a shape-memory alloy is formed at least at a portion, on which fitting during pressing is tight, of the molding surface of the fixed mold.

Preferably, a product shape of the workpiece has a hat-shaped section in a longitudinal direction and the shape-memory alloy layer is formed at each of portions of the molding surface of the fixed mold, the portions corresponding to a flange face and a top face of the hat-shaped section of the product shape of the workpiece.

Preferably, the shape-memory alloy layer formed on the molding surface of the fixed mold is formed by the superelastic alloy.

Advantageous Effects of Invention

With the metal mold for hot pressing according to the present invention, formation of a gap between the metal mold and the workpiece can be suppressed and uniform contact between the metal mold and the workpiece can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a metal mold for hot pressing.

FIG. 2 shows a lower mold having a molding surface provided with a shape-memory alloy.

FIG. 3 shows hot pressing by using the metal mold for hot pressing.

FIG. 4 shows other embodiment of the lower mold having the molding surface provided with the shape-memory alloy.

FIG. 5 shows an alternative embodiment of the metal mold for hot pressing.

DESCRIPTION OF EMBODIMENTS

A metal mold 1 is used for hot pressing for carrying out hot press molding of a workpiece 2 to manufacture a product. As shown in FIG. 1, the metal mold 1 includes a lower mold 10 and an upper mold 20 and sandwiches and presses the workpiece 2 between the lower mold 10 and the upper mold 20 to thereby mold the workpiece 2 into a predetermined product shape.

The workpiece 2 is a steel plate and heated to a temperature at which an austenite structure appears or a higher temperature and charged into the metal mold 1. The workpiece 2 is molded into the product shape in the metal mold 1 and rapidly cooled and quenched by being kept sandwiched between the lower mold 10 and the upper mold 20 for a certain time.

The workpiece 2 is molded into the product shape having a hat-shaped section in a longitudinal direction. In other words, the product shape of the workpiece 2 has a flange face and a top face formed as faces orthogonal to a pressing direction of the metal mold 1 and a wall face connecting the flange face and the top face. The wall face of the product shape is formed as a face substantially parallel to the pressing direction of the metal mold 1.

The flange face and the top face are not necessarily faces exactly orthogonal to the pressing direction and may be faces having certain degrees of inclination (e.g., faces inclined at 30 degrees or less with respect to the pressing direction) and may be faces having such degrees of inclination that the product having the flange face and the top face with this inclination can be used as a product having a hat-shaped section in general.

The lower mold 10 is formed as a fixed mold and fixed to a floor face or the like. A protruding portion protruding toward the upper mold 20 is formed on the lower mold 10 and the lower mold 10 is formed as a convex mold oriented upward.

The upper mold 20 is formed as a movable mold. The upper mold 20 is supported by a proper pressing machine and is movable with respect to the lower mold 10. A recessed portion conforming to the protruding portion of the lower mold 10 is formed on the upper mold 20 and the upper mold 20 is formed as a concave mold.

Surfaces of the lower mold 10 and the upper mold 20 facing each other are formed as molding surfaces and the workpiece 2 is molded according to shapes of the molding surfaces. In other words, the lower mold 10 and the upper mold 20 have the molding surfaces conforming to the product shape of the workpiece 2 and are disposed such that their molding surfaces face each other.

As shown in FIG. 2, a shape-memory alloy layer 30 is formed on the entire molding surface of the lower mold 10.

The shape-memory alloy layer 30 is an alloy layer formed integrally with the lower mold 10 by welding a shape-memory alloy to the surface of the lower mold 10 by laser welding or the like. The shape-memory alloy layer 30 is formed to have such thickness as not to affect molding performance of the metal mold 1.

As the shape-memory alloy forming the shape-memory alloy layer 30, generally used alloys such as a Ni-Ti alloy and an iron shape-memory alloy can be used. However, the shape-memory alloy forming the shape-memory alloy layer 30 needs to reach a transformation point to exert superelasticity at least at the time of hot pressing of the workpiece 2 by using the metal mold 1.

Here, as characteristics of the shape-memory alloy forming the shape-memory alloy layer 30, the shape-memory alloy preferably has such composition as to be able to easily reach a shape recovery temperature (e.g., after a few seconds) due to heat transfer from the workpiece 2 in a clamped state of the metal mold 1, and moreover, is preferably a superelactic alloy a shape recovery temperature of which is lower than or equal to a room temperature (about 20° C.).

With reference to FIG. 3, how the shape-memory alloy layer 30 works in hot pressing of the workpiece 2 by using the metal mold 1 will be described.

As shown in FIG. 3, by moving the upper mold 20 with respect to the lower mold 10 in a state in which the workpiece 2 is set, pressing of the workpiece 2 is started. Then, the upper mold 20 is further moved to reach a bottom dead point and is kept for a predetermined time in this state.

At this time, the shape-memory alloy layer 30 formed on the molding surface of the lower mold 10 is super elastically deformable due to the heat transfer from the workpiece 2 or setting of the shape recovery temperature. Therefore, portions on which fitting of the upper mold 20 is tight (portions corresponding to faces orthogonal to the pressing direction, e.g., a flange face and a top face in a product) are super-elastically deformed in response to pressing force received from the upper mold 20 and the workpiece 2. At the portions where the fitting is tight, the shape-memory alloy layer 30 is super-elastically deformed, strokes, and flows from a portion where the fitting is tight to a portion where the fitting is loose to thereby increase thickness of the portion where the fitting is loose such that a pressing load can be applied on the portion.

As described above, the superelastic deformation of the shape-memory alloy layer 30 can absorb the gap between the workpiece 2 and the lower mold 10. Moreover, because the molding surface of the lower mold 10 is deformed by the shape-memory alloy layer 30, how the lower mold 10 and the workpiece 2 fit on each other changes and the fitting of the workpiece 2 and the upper mold 20 on each other is improved as well. As a result, a gap becomes less likely to be formed between the workpiece 2 and the upper mold 20. In other words, by providing the shape-memory alloy layer 30 on the molding surface of the lower mold 10, it is possible to prevent formation of the gap between the metal mold 1 and the workpiece 2.

Particularly, if a product shape of the workpiece 2 has a hat-shaped section as in the present embodiment, it is known that wrinkles at portions to be bent largely (e.g., corner portions) or at shrink flanges are likely to occur. Even if a wrinkle occurs in such manner, the shape-memory alloy layer 30 is super-elastically deformed and, as a result, a portion of the molding surface of the lower mold 10 enters an inside of the wrinkle to thereby achieve uniform contact between the workpiece 2 and the metal mold 1. Furthermore, because tightness of the fitting on wall face portions of the product shape of the workpiece 2 can be increased, it is possible to shorten the time required for cooling of the workpiece 2. Therefore, in hot pressing the workpiece 2 by using the metal mold 1, it is possible to reduce a cycle time while securing quality.

After the end of the hot pressing by the metal mold 1, the pressing load by the upper mold 20 is released. Then, the shape-memory alloy layer 30 returns to its original shape due to superelasticity.

As shown in FIG. 4, the shape-memory alloy layer 30 provided to the lower mold 10 may be disposed partially instead of being disposed on the entire molding surface of the lower mold 10. To put it concretely, the shape-memory alloy layers 30 are disposed on portions, on which the fitting of the upper mold 20 is tight, of the molding surface of the lower mold 10, i.e., the portions corresponding to the flange face and the top face of the product shape of the workpiece 2. As a result, the shape-memory alloy layers 30 are super-elastically deformed at the portions on which the fitting of the upper mold 20 is tight, and the upper mold 20 strokes to increase tightness of the fitting on the wall face portions on which the fitting is loose. In other words, it is possible to uniformize the fitting of the metal mold 1 on the workpiece 2.

In this manner, it is possible to improve contact between the metal mold 1 and the workpiece 2 by disposing the shape-memory alloy layers 30 at least at the portions, on which the fitting is tight (the portions orthogonal to the pressing direction), of the molding surface of the lower mold 10.

As shown in FIG. 5, the shape-memory alloy layer 30 may be formed on the molding surface of the upper mold 20 as well as on the molding surface of the lower mold 10. By providing the shape-memory alloy layer 30 on the molding surface of the upper mold 20, it is possible to further improve contact between the metal mold 1 and the workpiece 2.

INDUSTRIAL APPLICABILITY

The present invention can be applied to the metal mold used for hot press molding for pressing a heated workpiece while, at the same time, cooling the workpiece.

DESCRIPTION OF NUMERALS

1: metal mold (metal mold for hot pressing), 2: workpiece, 10: lower mold, 20: upper mold, 30: shape-memory alloy layer 

1. A metal mold for hot pressing comprising a fixed mold and a movable mold disposed such that molding surfaces of the fixed mold and the movable mold face each other, the metal mold sandwiching a heated workpiece between the fixed mold and the movable mold to press the workpiece while, at the same time, cooling the workpiece by keeping the molding surfaces of the fixed mold and the movable mold in contact with surfaces of the workpiece, wherein a layer formed by a shape-memory alloy is formed at least at a portion, on which fitting during pressing is tight, of the molding surface of the fixed mold.
 2. The metal mold for hot pressing according to claim 1, wherein a product shape of the workpiece has a hat-shaped section in a longitudinal direction and the shape-memory alloy layer is formed at each of portions of the molding surface of the fixed mold, the portions corresponding to a flange face and a top face of the hat-shaped section of the product shape of the workpiece.
 3. The metal mold for hot pressing according to claim 1, wherein the shape-memory alloy layer formed on the molding surface of the fixed mold is formed by a superelastic alloy.
 4. The metal mold for hot pressing according to claim 2, wherein the shape-memory alloy layer formed on the molding surface of the fixed mold is formed by a superelastic alloy. 